Research On Fatigue Behavior Of Supercharged Boiler Drum At Startup And Steam Pressure Fluctuation

Supercharged boiler with its high heat load of furnace,high reliability,small size and good maintainability,etc.,has been as the main power boiler of the large ship steam power equipment.Compared with the power station boiler,the structure,heat transfer mode and load variation of the supercharged boiler are more complex,and it is difficult to arrange the measuring points on the drum wall,and then it becomes more difficult to get precise calculation of the temperature,stress and fatigue life of the boiler.In the typical process of supercharged boiler cold start,shutdown and large load disturbance,the pressure and temperature of boiler drum will produce violent fluctuations in a short period of time,generate cyclic thermal stress and mechanical stress with higher levels,and over time there will be hidden danger of plastic drum failure and fatigue damage,which seriously affect the safety of operators.Therefore,it is an urgent task to monitor the stress state and fatigue life of the supercharged boiler drum,in order to judge whether it can withstand the cycle heavy-loaded disturbance and what the endurance limits are.Currently,the research on fatigue life monitoring system for boiler drum used in power station has been matured day by day,but the study on the fatigue life monitoring of marine supercharged boiler drum is still left blank.This paper takes marine supercharged boiler as the research object.Firstly,the method of solving the drum temperature field,stress field and fatigue life is studied,and a mathematic model of the coupled solution for the forward and inverse problem of the drum temperature field is established.Secondly,the three-dimensional finite element model of the supercharged boiler drum is established by using the finite element software ANSYS.The thermal boundary conditions and mechanical boundary conditions are determined by the measured data,the heat transfer theory and the relevant design criteria.The drum temperature field,the thermal stress field,the mechanical stress field and the total stress field during the cold start process and vapor-filling process are calculated and analyzed,and the location of dangerous sections and dangerous points on the drum tubesheet are determined.The stress concentration factor at the edge of the tube hole is calculated according to the steady state process at the end of the cold start process.On the basis of previous research,the fatigue life calculation program of convexvapor was developed by using C ++ and Fortran programming,and a fatigue life monitoring system was developed for the drum of the supercharged boiler with the assistance of hardware facility on the test site.Through data tested to calibrate and verify,it proves that the accuracy is within the allowable range of deviation.Finally,carrying out the fatigue performance calculation and analysis of the different processes under different operating conditions of the supercharged boiler drum by using this system realizes the function of life monitoring and evaluation when the boiler drum is running.The results calculated show that descending tube bundle hole area and convective evaporative tube bundle hole area of the tube plate produced a higher level of stress in the cold start process and vapor-filling process.The low of total stress change of the dangerous point during the cold start process is that the dominant effect is the thermal stress before the refrigerant is saturated,and the thermal stress and the mechanical stress are superimposed after saturation.While the total stress of the dangerous point of the vapor-filling process is the dominant effect of mechanical stress.Too short time for cold start-up and too large pressure fluctuation on boiler drum during vapor-filling will cause separately the dramatic decrease of recycle times of cold start-up and the vapor charge-discharge processes,and the significant increase of fatigue loss of boiler drum.The result of the study in this paper reveals a significance of guiding in some way to optimize the control strategy of the boiler.